with the jeep running remove the oil fill plug, if you have bad blow by you will see it coming out, alot. if it isn't to bad check your pcv hose and see if its plugged, clean it good, replace pcv valve. . next step would be to remove the valve cover and check for sludge buildup, remove any sludge you find, clean the valve cover good especially the baffles. the 4.2 is known for getting a lot of sludge. mine was so bad you could hardly see the valve springs.